1. Field of the Invention
The present invention relates to an apparatus for controlling a continuously variable transmission and, more particularly, to a for controlling a range between upper and lower limits of a targeted speed of an engine, a continuously variable transmission in accordance with the running state of the vehicle, the driver's requirements, preferences, feelings, and so on.
2. Description of the Related Art
Conventionally, in the speed-change control of a continuously variable transmission mounted on a vehicle, a targeted value of an engine speed which is an input speed for the transmission, i.e., a targeted engine speed, is set on the basis of the engine performance. Upper and lower limits of the targeted engine speed are set centering on that value so as to stabilize control. Also, the number of speed changes is reduced so as to prevent a decline in the transmission efficiency during a speed change. This point can be understood from FIG. 17, a time t is plotted as the abscissa, and an engine speed N.sub.E/G is plotted as the ordinate. For instance, in the case of controlling the engine speed N.sub.E/G toward a targeted engine speed N* set on the basis of the maximum power curve of the engine, unless upper and lower limits are set for the targeted engine speed, the speed change is repeated frequently in the vicinity of the targeted engine speed N*, as shown by the dotted line in the drawing so as to bring the engine speed N.sub.E/G toward the targeted engine speed N*. Accordingly, the speed-change control becomes unstable, and the transmission efficiency of the transmission declines during the speed changes. To cope with this situation, if upper and lower limits N*.sub.max and N*.sub.min are provided for the targeted engine speed N*, the number of speed changes decreases as shown by the solid line in the drawing, control becomes stable, and it is possible to prevent a decline in the transmission efficiency during the speed changes. Thus, by providing N*.sub.max and N*.sub.min, the speed change is suspended when the engine speed N.sub.E/G reaches N*, and the speed change is not effected during N*.sub.min &lt;N.sub.E/G &lt;N*.sub.max so as to secure control stability and prevent the decline in transmission efficiency during speed changes.
However, as a method of determining a range between the upper and lower limits of the targeted engine speed, N.sub.H =N*.sub.max -N*.sub.min, if N.sub.H is made large, the number of speed changes decreases and it is possible to prevent the decline in transmission efficiency during speed changes, and stability of control improves. However, if N.sub.H is large, the difference between N* and N.sub.E/G becomes large, so that the power performance and fuel consumption performance (in a case where a best fuel consumption curve is selected) deteriorates. On the other hand, if N.sub.H is made small, control stability deteriorates, but since N* is traced faithfully, the power performance and fuel performance improve. Conventionally, the characteristics have been determined in a fixed manner by placing emphasis on either one of the above-described alternative methods. In either case, however, the above-described drawbacks have been experienced.